1. Field of the Invention
The present invention pertains to acoustic well logging and more particularly to an acoustic isolator for use in an acoustic logging system.
2. Related Prior Art
Acoustic logging tools for measuring properties of the sidewall material of both cased and uncased boreholes are well known. Essentially such tools measure the travel time of an acoustic pulse propagating through the sidewall material over a known distance. In some studies, the amplitude and frequency of the acoustic pulse, after passage through the earth, are of interest.
In its simplest form, an acoustic logger consists of one or more transmitter transducers that periodically emit an acoustic signal into the formation around the borehole. One or more receiver transducers, spaced apart by a known distance from the transmitter, receives the signal after passage through the surrounding formation. The difference in time between signal transmission and signal reception divided into the distance between the transducers is the formation velocity. If the transducers do not contact the borehole sidewall, allowance must be made for time delays through the borehole fluid.
Throughout this disclosure, the term “velocity”, unless otherwise qualified, shall be taken to mean the velocity of propagation of an acoustic wavefield through an elastic medium. Acoustic wavefields propagate through elastic media in different modes. The modes include: compressional or P-waves, wherein particle motion is in the direction of wave travel; transverse shear or S-waves, which, assuming a homogeneous, isotropic medium, may be polarized in two orthogonal directions, with motion perpendicular to the direction of wave travel; Stonley waves, which are guided waves that propagate along the fluid-solid boundary of the borehole; and compressional waves that propagate through the borehole fluid itself. There also exist asymmetrical flexural waves as will be discussed later.
P-waves propagate through both fluids and solids. Shear waves cannot exist in a fluid. Compressional waves propagating through the borehole fluid may be mode-converted to shear waves in the borehole sidewall material by refraction provided the shear-wave velocity of the medium is greater than the compressional-wave velocity of the borehole fluids. If that is not true, then shear waves in the sidewall material can be generated only by direct excitation.
Among other parameters, the various modes of propagation are distinguishable by their relative velocities. The velocity of compressional and shear waves is a function of the elastic constants and the density of the medium through which the waves travel. The S-wave velocity is, for practical purposes, about half that of P-waves. Stonley waves may be somewhat slower than S-waves. Compressional wavefields propagating through the borehole fluid are usually slower than formational shear waves but for boreholes drilled into certain types of soft formations, the borehole fluid velocity may be greater than the sidewall formation S-wave velocity. The velocity of flexural waves is said to approach the S-wave velocity as an inverse function of the acoustic excitation frequency. Some authors refer to flexural waves as pseudo-Raleigh waves.
In borehole logging, a study of the different acoustic propagation modes provides diagnostic information about the elastic constants of the formation, rock texture, fluid content, permeability, rock fracturing, the goodness of a cement bond to the well casing and other data. Typically, the output display from an acoustic logging tool takes the form of time-scale recordings of the wave train as seen at many different depth levels in the borehole, each wave train including many overlapping events that represent all of the wavefield propagation modes. For quantitative analysis, it is necessary to isolate the respective wavefield modes. S-waves are of particular interest. But because the S-wave arrival time is later than the P-wave arrival time, the S-wave event often is contaminated by later cycles of the P-wave and by interference from other late-arriving events. Therefore, known logging tools are designed to suppress undesired wave fields either by judicious design of the hardware or by post-processing using suitable software. Both monopole and dipole signals may be transmitted and received using appropriately configured transducers. Because the systems measures signal transit time, it is crucial that the spatial relationship between the transmitter and receivers remain essentially constant during logging. For monopole signals, the distance between transmitter and receivers should remain essentially constant. For dipole signals, both the distance and rotational orientation between transmitters and receivers should remain essentially constant during logging.
As is well known, the acoustic transmitter and the acoustic receivers are mounted at opposite ends of a logging sonde. The body of the sonde is usually of a suitable metal such as stainless steel or the like which is acoustically conductive. Therefore, in order to prevent unwanted acoustic energy traveling up the sonde from interfering with desired acoustic energy propagating through the formation, is it required that an acoustic isolator be inserted in the sonde between the transmitter and the receivers.
In addition, the deployment of acoustic tools using coiled tubing or drill pipe has increased the loading, both axial and rotational, on the acoustic sonde. For example, in highly deviated or horizontal wellbores, the logging tool may be deployed with drill pipe. The drill pipe may be slowly rotated to reduce the frictional resistance between the pipe and the borehole wall while deploying or extracting the logging tool. Residual axial and/or rotational loading may be transferred through the acoustic logging tool, even during the logging sequence.
Prior art isolators, commonly used with wireline deployment, have proven to be fragile or to deform excessively, either axially or rotationally, under the high loading encountered in pipe conveyed logging. For example, U.S. Pat. No. 3,191,141, issued Jun. 22, 1965 to Schuster, describes a slotted sleeve isolator placed between a transmitter and a receiver. The slotted arrangement forms a serpentine travel path for acoustic wave energy, both delaying and attenuating the wave. The slotted sleeve is often adequate for tools with only monopole transmitters, but has often proved inadequate for dipole or other forms of multipole transmissions. In addition, the slotted configuration has proven to be fragile in high axial loading situations.
U.S. Pat. No. 4,872,526, issued Oct. 10, 1989 to A. Wignall et al., U.S. Pat. No. 5,728,978 to Roberts et al., and U.S. Pat. No. 5,229,553 to Lester et al., all use a plurality of captured elastomeric, typically rubber, elements to provide through-tool signal attenuation. The elastomeric elements unacceptably deform both axially and rotationally under the high loading of pipe conveyed logging. This deformation results in unacceptable errors in the resulting logs, especially from multi-pole sources.
There is a need for an acoustic isolator that will be sufficiently flexible to pass through deviated boreholes yet sufficiently rigid to provide axial and rotational dimensional stability between the transmitters and receivers of the logging tool.